I have a Racore Drive II and I've been looking for a way to render its parallel port bidirectional. Here's how you can do it:

First, you have to unsolder the 74LS273 chip that is near the DB25 connector (don't confuse this chip with the 74HC273 that is near the top edge of the board). Since you could damage your board if you heat it too much with your soldering iron, I suggest you cut the chip's legs first using a small wire cutter. Believe me, it will be a lot easier to unsolder each pin individually than trying to salvage the chip. Besides, this chip can be had for less than a dollar; so I don't see why you would want to save it anyway.

The next step is to replace the 74LS273 with a 74LS374. While you could just solder the new chip in place of the old one, I suggest you solder a 20-pin socket instead. This way, if the parallel port ever suffer damage in the future (e.g. short circuit), it will be easy to swap the chip.

The final step is to apply the "standard" mod, i.e. cut the trace that goes to pin 1 of the 74LS374 (that is, the 20-pin socket), and then run a jumper from this pin to pin 15 of the 74LS174.

DISCLAIMER: I have not tested this mod extensively. I modified my board, and then used Mike's BASIC program (http://www.brutman.com/PCjr/parallel_port.html) to confirm the presence of a bidirectional port. For now, I still use MS Interlink to connect my PCjr to a modern PC, but this software doesn't benefit from using a true bidirectional port; it transfers data in nibble (4-bit) mode. Besides, byte-mode data transfer would require a different cable.